Manufacture of bleaching powder



atented eph 5, i933 MANUFACTURE OF BLEACHING POWDER Charles Carter, Hunts Cross, Liverpool, England, assignor to Imperial Chemical Industries company Limited, London,

England, a British No Drawing. Application November 17, 1930, Serial No. 496,332, and in Great Britain November 26, 1929 Claims.

This invention relates to an improvement in the process of manufacturing bleaching powder (either ordinary or the socalledrtropical bleaching powder) by interaction of partially or wholly 5 slaked lime or partially chlorinated lime with a counter current of diluted chlorine in an agitating apparatus, adapted for continuous production. The conditions here are quite dilierent from those of processes such as the Weldon chamber process, in which strong chlorine is used without agitation of the solid in counter current and my invention relates to continuous processes of the type set forth.

It is well known that bleaching powder is conveniently prepared by agitation of slaked lime in counter current to chlorine gas diluted with more or less air, preferably in a rotary tube. Hitherto the output of any particular rotary tube has been limited, not only by the speed of the reaction, which normally requires of the order of 8 hours contact between lime and chlorine, but by the rate of dispersal of heat. The only means of dispersal of heat have been, so far, heat radi ation or external cooling, and circulation of a large volume of air which may or may not be refrigerated before entering the plant, and which leaves the plant at an elevated temperature. Heat is also absorbed in the evaporation of part of the water liberated in the reaction. Up to now any attempt to increase the output to any considerable extent has resulted in weak bleaching powder. I

In view of the desirability of ultimately producing a product low in moisture, it has hitherto been the tendency to exclude moisture as far as possible and it has been proposed to dry the circulating air.

I attribute the failure to produce until now strong bleach on increased output not only to decomposition, caused by excessive temperature, but also to premature drying of the partly chlorinated lime to such a stage that further chlorination, if it proceeds at all, is extremely slow.

I have now found that it is possible to produce r full strength bleaching powder very rapidly in an agitating apparatus, by introducing water into the apparatus during the operation. The water so introduced prevents the premature drying the further benefits referred to above. ter is preferably introduced in an atomized condition, by means of any suitable spray, which protrudes into a rotary tube or other agitating apparatus and delivers the water preferably at the zone of greatest reaction. The quantity of water used will be adjusted according to, among other conditions, the output desired, thevolume of circulated gas, and the proportion of water desired in the resulting bleaching powder. The water spray has'the additional advantage that it reduces the quantity of dust carried off in the exit gases.

When using liquid water, I may employ slaked or partially slaked lime or partially chlorinated ime.

As examples of the quantities of water required to be used'I give the following.

1. In a cast iron rotary tube of dimensions 20 ft. by 3 ft. capable normally of an output of less than 1 ton per day I have obtained an output of 2.6 tons bleaching powder per day by spraying in water at the rate of .23 tons per day.

2. In the same rotary tube an output of 4 tons bleaching powder per day was obtained by spraying in .68 tons of water per day.

3. In a it. cylinder capable normally of an output of 4 to 5 tons bleaching powder per day from already partly chlorinated lime averaging 8 to 10% available chlorine, the output was increased to 8 to 10 tons per day by spraying in water at a rate of about .3 tons water per day, the lime feed, the speed of the cylinder, and the chlorine and airflow being doubled.

Needless to say I do not confine myself to these examples. As mentioned above, the quantity of water used will depend on many other conditions. As seen from the above examples even the proportion of water to lime will not be constant, since it will vary with different designs of plant, and will, for any particular plant, increase as the out- The waput is increased, and will also depend to some exprinciple given above, so that the quantity of water evaporated absorbs in its evaporation the surplus heat, thus preventing premature drying and excessive temperature.

Some of the advantages of the "invention can 3105 be obtained by use of high moisture lime, that is to say lime which contains a higher proportion of water than corresponds to that of calcium hydrate, but I prefer the introduction of liquid water which controls the temperature, accelerates the reaction and reduces the amount of dust carried off.

The use 01 liquid water is preferred, steam being very much less efficient for mypurpose since it exerts no cooling action such as that resulting from the evaporation of sprayed liquid water.

The use of high moisture lime suffers from certain disadvantages chief amongst them being the diliiculty of feeding it mechanically at a uniform rate.

According to a further modification of the present invention I treat slaked lime or partially chlorinated lime with hot gases containing chlorine whilst adding water to prevent premature drying of the material. In this way it is possible to produce directly a high-strength bleaching powder suitable for use in tropical countries" (eJg. a product containing say 1 to 1%;% only of moisture).

It has been proposed to produce the so-called tropical bleaching powder by drying "previously made bleaching powder with hotair in a second operation (British 'Patent"2 i2,805) :or by direct production oi one operation, using circulation of a large'volume of dilute chlorine whichpassisted by the heat of reaction, allows of removal of-'moisture bythe. circulating gas.

I have attempted to use hot inlet gases for the purpose'of effecting chlorination and drying in one operation but the resultant product wasweak, ii'properly' dried. I believe'thisresult' is dueto premature drying of the bleach. "My present invention allows me to obtain the advantage of using hot inlet gasand yet "by injecting water into: the apparatus (which maybe a rotary tube) I avoid this premature drying and thus secure thorough chlorinationof the solid, which as it proceeds along the .tube is dried byithe hotgas.

The injection of cold waterhas the remarkable effect of allowing of the production of a highstrength dried product in a single operation because it allows of the use of hot inlet gases without the disadvantages hitherto attendant thereon.

Moreover the water, in addition to preventing premature drying and lowering the temperature locally (by absorbtion of heat due to evaporation) appears to act as a catalyst in the reaction. I

less desirably, inject steam or use lime containing excess of moisture but I prefer the use of liquid water for the reasons stated.

The quantity of water added is preferably above 0.5%calculated on the weight of thebleaching powder produced, but I. prefer to use quantities of the. order of 2 to 3%. If precautions are taken to remove 'dust, (which'is necessary owing to the high 1 velocity of .the gases) a very much larger quantity of water may be used, with great increase in. the speed of the reaction.

With about 23% of water, it is possible to make 'When making so-called tropical bleaching .powder, it is believed novel to obtain this prodnot in one operation by the use of hot inlet gas. Considerable increases of output are obtained according tothe invention though obviously the output of tropical bleaching powder will be less than that of the ordinary product, since it is necessary to chlorinate and dry in one operation.

My object in performing the reaction in presence of an excess of moisture is not to raise the percentage of available chlorine in the final product, (since my product, like other commercial bleaching powders, contains substantially less than the theoretically possible quantity of ,avail able chlorine) but toincrease the possible rate of reaction by allowingior'local dissipation of undesirableheat and probably by catalytic acceleration of the chlorination.

The term chlorinatable lime is employed to include both partially and wholly slaked lime and partially chlorinated lime.

We use the term hot gas in the claim to refer to gas heated above 50 C. which is'oiten'regarded in this art as a'ternperature which should be avoided. to'prevent decomposition of the product.

I declare'that'whatl claim is:

1. The process or" making bleaching powder by circulating a large volume of dilute chlorine in counter current with chlorinatable lime and simultaneouslyintroducing liquid water at'the chlorinating zone.

2. The process of making bleaching powder by circulating a large-volume of dilute chlorine in counter current with agitated slaked lime and simultaneously introducing a spray of liquidwater at the chlorinating zone.

3. The continuous process of producing bleaching powder by chlorinatinga solid 'chlorinatable material which includes the step of injectingliquid' water into the reactionzone. 4. In'the'pro'cess of making bleaching powder bycirculating a large'volurne of dilute chlorine in counter curren with agitated chlorinatablc limeythestep of admitting'liquidwaterto the chlorinating zone in a quantity of at least"2-3 H2O of the weight of bleaching powderformed.

"5. The process of making tropical bleaching powder by circulating a large volume of hotand substantially dry dilute chlorine in countercurrent with chlorinatable lime agitatingthe lime and simultaneously introducing liquid water at 

